1. Field of the Invention
To produce casting molds and cores, frequently binder systems on the basis of polyurethane are used. These are two-component systems, one component of which consists of polyols with a minimum of two OH groups in the molecule and the other of polyisocyanates with a minimum of two NCO groups in the molecule. These two components, in dissolved form, are added to a basic granular molding material (in most cases sand) and are subjected to a curing reaction by adding a catalyst.
2. Description of the Related Art
In a typical example of such systems, the polyol is a precondensate of phenol or phenol compounds with aldehydes which contains free OH groups (hereinafter referred to as "phenolic resin"), and the polyisocyanate is an aromatic polyisocyanate, such as diphenylmethanediisocyanate. Tertiary amines are used as catalyst. Depending on whether the cold-box process or the nobake process is used, the catalyst, in combination with the remaining ingredients of the binder system, is added either immediately prior to processing the molding material mixture or after the molding material mixture, which is initially produced without catalyst, has been added into a mold in which the mixture is gassed with gaseous amine.
In this type of system, solvents are required to ensure that during mixing with the basic molding material, the components of the binding agent are maintained at a sufficiently low viscosity. This is particularly true with respect to phenolic resins which, due to their higher viscosity always require a solvent, but it also applies to polyisocyanates. One problem encountered in this context is that the two binder components require different types of solvents. Thus, as a rule, nonpolar solvents work well with polyisocyanates but are not readily compatible with phenolic resins, and the reverse applies to polar solvents. In practice, it is therefore common to use mixtures of polar and nonpolar solvents which are balanced specifically for the binder system used. In this context, it should be ensured that the boiling range of the individual components of this mixture is not too low so that the solvent does not turn prematurely ineffective due to evaporation.
The nonpolar solvents preferably used so far were high-boiling aromatic hydrocarbons (mainly in the form of mixtures) with a boiling range above approximately 150.degree. C. at normal pressure, and the polar solvents used were, among other things, certain sufficiently high-boiling esters, such as the "symmetrical" esters described in the German Patent Specification No. 2,759,262, the acid residue and the alcohol residue of which contain a relatively large number of C atoms within the same range (approximately 6-13 atoms).
In spite of all the advantages of polyurethane binders for foundry technology, these binders have one serious drawback in that they are responsible for evaporations and the gas evolution in the working place, which, in most cases, cannot be prevented by protective measures, such as fume hoods, or similar devices. As a result of the fact that, in the meantime, it was possible to reduce the residual content of free formaldehyde and free phenol, the development in the area of resins has led to products which cause very low workplace exposure; and even with respect to the esters which, by nature, have a disagreeable smell, it has been possible to improve the situation markedly by the use of the symmetrical esters mentioned above, but what remains is the problem of exposure to the high-boiling aromatic hydrocarbons in the working place, which so far could not be dispensed with. These aromatic hydrocarbons are generally alkyl-substituted benzenes, toluenes, and xylenes. To ensure the highest possible boiling point, however, they may, in addition, also contain compounds with condensed benzene rings, i.e., naphthalene, etc., which are substances considered hazardous to human health and which are released not only after casting but already during the production of the molding material mixtures.